Naphthenic acids are carboxylic acids that occur in most crude oils as trace components and in some, biodegraded oils in significantly greater concentrations. Total acids in crude oils is commonly semi-quantified by titration with KOH and expressed in terms of total acid number (TAN). Conventional TAN measurements are not precisely a measure of total acids in a crude oil, but a measure of the amount of KOH needed to achieve the deflection (neutralization) point. Accordingly, TAN is an approximation of the amount of naphthenic acids. The acidity of high TAN oils may cause emulsion and corrosion problems in both production and refining. Solid deposits, recently identified as calcium naphthenates, can result in substantial damage and lose of production.
Under certain conditions, the naphthenic acids present in acidic crude oil will precipitate with Ca2+ ions that are present in the co-produced water to form calcium naphthenate solids. Other cations are involved to a lesser extent forming a variety of metal naphthenates (e.g., ferrous iron and magnesium). This solid precipitation accumulates predominantly in oil-water separators and desalters, but naphthenates also can deposit in subsea, topside, or surface facilities and pipelines.
A great deal of research has been pursued to characterize the naphthenic acid responsible for the calcium deposits. It has been recently determined that a specific family of high molecular weight tetracarboxylic acids, termed ARN Acids, appears to be the major constituent responsible for the calcium naphthenate deposits (ARN is not an acronym, but is Old Norwegian for “eagle”). ARN acids are high molecular weight molecules with four carboxylic acid groups, each at the end of a long aliphatic chain, forming a four-fingered molecule with polar tips. The ARN acids are a specific family of ˜C80 tetracarboxylic acids. A majority of the ARN acids have a molecular weight ranging from about 1228 to about 1236 atomic mass units (amu) with one of the main acids having a molecular weight of 1232 amu and a molecular formula of C80H142O8. The ARN acids do not have an aromatic or alkenes function present and quaternary carbons do not exist. The ARN acids can have 4-8 sites of unsaturation (or 4-8 cyclopentyl rings) and are believed to be derived from archaeal C80 lipids.
The proposed structure of the major ARN acid is 6:17,10:18,10′:18′,6″:17″,10″:18″,10″:18″)-hexacyclo-20-bis-16,16″-biphytane-1,1′,1″,1′″-tetracarboxylic acid. The molecule contains two biphytanyl diacids, each with three pentacyclic rings joined together by a linkage at the C20 methyl groups, as described in Lutnaes B. F., Brandal Ø., Sjöblom J., and Krane J. (2006) Archaeal C80 isoprenoid tetraacids responsible for naphthenate deposition in crude oil processing. Organic & Biomolecular Chemistry 4, 616-620, incorporated by reference in its entirety herein.
The structure of a representative archaeal C80 isoprenoid tetra-acid is:

The four carboxylic acid groups afford the molecule unusually high reactivity. These four carboxylic groups tend to create polymeric salt when coordinated with divalent metal ions. This woven polymeric-like structure yields a very sticky deposit that hardens upon contact with air.
A method for selectively isolating carboxylic acids from oils and calcium naphthenates using an Acid-Ion Exchange Resin procedure has been described in Mediaas et al. (2003) The Acid IER Method—a Method for Selective Isolation of Carboxylic Acids from Crude Oils and Other Organic Solvents, Society of Petroleum Engineers Paper 80404. The ion-exchange method is suitable when dealing with cleaning up Ca-naphthenate precipitates as most of the material is composed of ARNs. However, this method is likely not suitable for industrial scale separation because the resins will eventually foul with the associated hydrocarbons/aspahltenes. Furthermore, the use of ion-exchange resins on an industrial scale level may be cost prohibitive.
A need therefore exists for alternative and more efficient and effective methods to isolate and extract high molecular weight naphthenic tetra-acids from calcium naphthenate salts.